In the refining of crude oil, conventional processing to recover fractions suitable for upgrading in various refinery processing operations begins with distillation, wherein the crude oil is first distilled in an atmospheric distillation tower, with residual material from the bottom of the distillation tower often further separated in a vacuum distillation tower. In this operation, gas and gasoline generally are recovered as overhead products of the atmospheric distillation tower, heavy naphtha, kerosene and gas oils are taken off as distillate side streams and the residual material is recovered from the bottom of the tower as reduced crude. The reduced crude is often charged to a vacuum distillation tower. The vacuum distillation step in lube refining provides one or more raw lube stocks within the boiling range of about 550.degree. F. to 1050.degree. F., as well as the vacuum residuum byproduct.
In lube refining, following vacuum distillation, each raw stock is extracted with a solvent, e.g. furfural, phenol or chlorex, which is selective for aromatic hydrocarbons, removing undesirable components. The vacuum residuum usually requires an additional step, typically propane deasphalting, to remove asphaltic material prior to solvent extraction. The products produced for further processing into base stocks are known as raffinates. The raffinate from solvent refining is thereafter dewaxed and then processed into finished lube base stocks.
The solvent extraction step separates hydrocarbon mixtures into two phases; the previously described raffinate phase which contains substances of relatively high hydrogen to carbon ratio, often called paraffinic type materials, and an extract phase which contains substances of relatively low hydrogen to carbon ratio often called aromatic type materials. Furfural is typical of a suitable solvent extraction agent. Its characteristics permit use with both highly aromatic and highly paraffinic oils of wide boiling range. Diesel fuels and light and heavy lubricating stocks are often refined with furfural.
While the furfural solvent extraction unit raffinate goes on to further processing, the extract from the operation often finds utility in a broad range of industrial applications. Applications for these aromatic extracts often vary according to the particular properties of the extracts, these properties largely a function of the feedstock used and unit conditions. For example, as described in "A New Look at Oils in Rubber" by H. F. Weindel and R. R. Terc, Rubber World, December, 1977, these extracts often find further utility as low and high viscosity aromatic extender oils for rubber processing. Bright stock extracts (BSE's), obtained by solvent-refining deasphalted vacuum resids during the production of bright stocks, are also useful in rubber processing and find utility as ink oils as well. Like the lighter aromatic extracts, BSE's possess excellent solvent characteristics which lend themselves to great potential utility.
Besides having utility as a feedstock to the solvent extraction unit, the raffinate stream of the deasphalting unit can find further utility as a specialty oil. Depending on its characteristics, this stream, also known as deasphalted oil (DAO), can find utility as an extender oil for rubber processing, as an ink oil, etc.
In recent years, concerns have arisen regarding the potential hazards associated with the use of various lubricating oils, middle distillates, aromatic oils and other hydrocarbon-based products containing polynuclear aromatics (PNA's), since certain of these compounds have been shown to cause cancer in humans and laboratory animals following exposure thereto. Previous studies of the higher boiling fractions recovered from vacuum distillation and processed to formulate engine oils and other lubricants have established a fairly consistent pattern of the types of petroleum-derived materials which cause tumors in laboratory animals. Extensively treated oils, such as those treated by solvent refining and severe hydroprocessing are known to only have trace amounts of PNA's. As such, these materials are generally not tumorigenic; while materials having high PNA levels, especially those compounds having three or more rings, are.
Concerning the use of various aromatic oils, DAO's and BSE's, U.S. Pat. No. 4,321,094, notes at col. 2, lines 9-14, that "many printing ink oils still contain proportions of aromatic hydrocarbons which either are proven to be carcinogenic, such as benzene, or are believed to be carcinogenic, such as toluene and polycyclic compounds. Clearly elimination of these from an ink would be desirable for health reasons." As a result of these concerns, many refiners are no longer willing to supply DAO's or aromatic extracts, including BSE's, for these speciality applications. Those refiners that continue to market these products must provide labels outlining the potential risks associated with the use of these products. This has led to the development and selection of alternate materials for applications previously fulfilled by aromatic oils, as evidenced by U.S. Pat. Nos. 4,321,094 and 4,519,841. The use of these alternative solvents often carries with it the penalty of higher cost and inferior finished product quality.
Further, as disclosed in U.S. Pat. No. 4,321,094, an inventor of which is also a co-inventor of the present invention, certain middle distillates used as specialty oils possess the potential for significant human exposure due to the nature of their industrial applications. While such straight-run middle distillates boil below 700.degree. F. and typically contain only small levels of PNA compounds, they would not be expected to cause tumors in tests conducted on laboratory animals. However, experiments using laboratory animals have shown this to not be the case.
To determine the relative mutagenic activity of a petroleum-based product, a reliable test method for assaying such activity in complex hydrocarbon mixtures is required. A highly reproducible method showing strong correlation with the carcinogenic activity index of hydrocarbon mixtures is disclosed in U.S. Pat. No. 4,499,187, which is incorporated by reference in its entirety. From the testing of hydrocarbon samples as disclosed in U.S. Patent No. 4,499,187, a property of the sample, known as its Mutagenicity Index (MI) is determined. Hydrocarbon mixtures exhibiting MI's less than or equal to 1.0 are known to be non-carcinogenic, while samples exhibiting MI's equal to about 0.0 are known to be completely free of mutagenic activity.
U.S. Pat. No. 5,034,119, an co-inventor of which is also a co-inventor of the present invention, discloses a process for producing non-carcinogenic bright stock extracts and deasphalted oils from reduced hydrocarbon feedstocks. Such non-carcinogenic products are produced by establishing a functional relationship between mutagenicity index and a physical property correlative of hydrocarbon type for the bright stock extract or deasphalted oil and determining a critical physical property level which, when achieved, results in a product having a mutagenicity index of less than about 1.0. Process conditions are established so that a product stream achieving the desired physical property level can be produced. Non-carcinogenic bright stock extracts or deasphalted oils are then processed utilizing the conditions so established.
Despite these advances in the art, a need exists for a process for reducing the mutagenicity of a broad range of petroleum-based products.